In the recent drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The background supporting such a rapid advance is a reduced wavelength of the light source for exposure. The change-over from i-line (365 nm) of a mercury lamp to shorter wavelength KrF excimer laser (248 nm) enabled mass-scale production of dynamic random access memories (DRAM) with an integration degree of 64 MB (processing feature size ≦0.25 μm). To establish the micropatterning technology necessary for the fabrication of DRAM with an integration degree of 256 MB and 1 GB or more, the lithography using ArF excimer laser (193 nm) is under active investigation. The ArF excimer laser lithography, combined with a high NA lens (NA 0.9), is considered to comply with 65-nm node devices. For the fabrication of next 45-nm node devices, the F2 laser lithography of 157 nm wavelength became a candidate. However, because of many problems including a cost and a shortage of resist performance, the employment of F2 lithography was postponed. ArF immersion lithography was proposed as a substitute for the F2 lithography (see Proc. SPIE Vol. 4690, xxix, 2002).
In the ArF immersion lithography, the space between the projection lens and the wafer is filled with water and ArF excimer laser is irradiated through the water. Since water has a refractive index of 1.44 at 193 nm, pattern formation is possible even using a lens with NA of 1.0 or greater. The theoretically possible maximum NA is 1.44. The resolution is improved by an increment of NA. A combination of a lens having NA of at least 1.2 with ultra-high resolution technology suggests a way to the 45-nm node (see Proc. SPIE Vol. 5040, p. 724, 2003).
The ArF immersion lithography has a possibility that water-soluble components in the resist film be leached in immersion water during exposure. Specifically an acid generated during exposure and a basic compound previously added to the resist material can be leached in immersion water. As a result, pattern profile changes and pattern collapse can occur. It is also pointed out that if the resist film is less water repellent, water droplets remaining on the resist film after scanning, though in a minute volume, can penetrate into the resist film to generate defects. It was then proposed to provide a protective coating between the resist film and water to prevent resist components from being leached out and water from penetrating into the resist film, the process being referred to as “topcoat process.” See 2nd Immersion Workshop: Resist and Cover Material Investigation for Immersion Lithography, 2003.
In the ArF immersion lithography using a topcoat, a protective coating material which is soluble in alkaline developer is advantageous. This eliminates the step of stripping off the protective coating, offering great cost and process merits. Thus, great efforts have been devoted to develop water-insoluble resist protective coating materials, for example, resins having alkali-soluble units such as fluorinated alcohol, carboxyl or sulfo groups, as described in JP-A 2006-91798.
On the other hand, a process for preventing resist components from being leached out and water from penetrating into the resist film without a need for a protective coating material has also been developed, the process being referred to as “topcoatless process”, as described in JP-A 2007-187887. In the topcoatless process, an alkali-soluble hydrophobic polymer is added to the resist material as a surfactant, whereupon the hydrophobic compound is segregated at the resist surface during resist film formation. The process is thus expected to achieve equivalent effects to the use of resist protective coating material. Additionally, the process is economically advantageous over the use of a resist protective film because steps of forming and removing the protective film are unnecessary.
In either of the topcoat and topcoatless processes, the ArF immersion lithography requires a scanning speed of about 300 to 700 mm/sec in order to gain higher throughputs. In the event of such high-speed scanning, if the water repellency of the resist or protective film is insufficient, water droplets may be left on the film surface after scanning. Residual droplets may cause defects. To eliminate such defects, it is necessary to improve the water repellency of the relevant coating film and the flow or mobility of water (hereinafter, water slip) on the film. The film material must be designed so as to increase the receding contact angle (see 2nd International Symposium on Immersion Lithography, 12-15 Sep., 2005, Defectivity data taken with a full-field immersion exposure tool, Nakano et al). In connection with such polymer design, it is reported that introduction of fluorine is effective for improving water repellency, and formation of micro-domain structure by a combination of different water repellent groups is effective for improving water slip. See Progress in Organic Coatings, 31, p. 97 (1997).
One exemplary material known to have excellent water slip and water repellency on film surface is a copolymer of α-trifluoromethylacrylate and norbornene derivative (Proc. SPIE Vol. 4690, p. 18, 2002). While this polymer was developed as the resin for F2 (157 nm) lithography resist materials, it is characterized by a regular arrangement of molecules of (highly water repellent) α-trifluoromethylacrylate and norbornene derivative in a ratio of 2:1. When a water molecule interacts with methyl and trifluoromethyl groups, there is a tendency that the orientation distance between water and methyl is longer. A resin having a regular arrangement of both substituent groups is improved in water slip because of a longer orientation distance of water. In fact, when this polymer is used as the base polymer in a protective coating for immersion lithography, water slip is drastically improved (see US 20070122736 or JP-A 2007-140446). Another example of the highly water repellent/water slippery material is a fluorinated ring-closing polymerization polymer having hexafluoroalcohol groups on side chains. This polymer is further improved in water slip by protecting hydroxyl groups on side chains with acid labile groups, as reported in Proc. SPIE Vol. 6519, p. 651905 (2007).
Although the introduction of fluorine into resins is effective for improving water repellency and water slip, the introduction of extra fluorine can induce new defects known as “blob defects”. Blob defects are likely to form during spin drying after development, particularly when the film has a high surface contact angle after development. One approach for suppressing blob defects is by introducing highly hydrophilic substituent groups (e.g., carboxyl or sulfo groups) into a resin to reduce the surface contact angle after development. However, since these groups serve to reduce the water repellency and water slip of the resin, this approach is not applicable to high-speed scanning. There is a desire to have a resin material which can minimize blob defects while maintaining highly water repellent and water slip properties during immersion lithography.
The highly water repellent/water slippery materials discussed above are expected to be applied not only to the ArF immersion lithography, but also to the resist material for mask blanks. Resist materials for mask blanks are subject to long-term exposure in vacuum. It is pointed out that sensitivity variations or profile changes can occur as an amine component in the resist material is adsorbed to the resist film surface during the long-term exposure. It was then proposed to add a compound having surface active effect to modify the surface of a resist film for preventing adsorption of amine to the resist film.